Of the factors influencing the fuel economy of semi-trucks, aerodynamics is the field in which the greatest improvements might be most readily made. Of the two types of aerodynamic drag, friction drag and pressure drag, pressure drag has a particularly significant impact on heavy trucks, accounting for as much as 90% of drag force. On a standard, unmodified truck and trailer, approximately one third of this pressure drag is caused by the vehicle undercarriage.
When the elements which cause drag are essential structural components, they cannot be removed, displaced, or dramatically altered. Instead, aerodynamic fairings can be attached to the vehicle to improve airflow, thereby reducing drag and consequently fuel consumption.
The exposed crossmembers on the underside of a trailer are prohibitive to smooth, stable airflow. These I beams span the width of the vehicle, but are spaced approximately one foot apart, leaving regularly alternating gaps along the length of the trailer. Closing or otherwise covering these gaps would transform the uneven underside of the trailer into a smooth, flat surface, improving aerodynamic efficiency and minimizing turbulence.
As crossmembers are a key structural component of a trailer and must support tremendous weight, any aerodynamic attachments applied to the crossmembers must not undermine their structural integrity and will ideally leave them completely intact. Producing holes in the crossmembers could weaken them and increase the incidence of cracks or failures. Furthermore, the possibility of structural failure makes it preferable for the crossmembers to be at least partly visible, such that they can be inspected for weaknesses even with aerodynamic fittings installed.
It is also preferable that any such attachments do not prevent someone from accessing or servicing the underside of the trailer. If possible, these fairings should be readily removable, in whole or in part, so that one may conduct an inspection or perform maintenance as needed.